1. Field of the Invention
The present invention relates to a flow rate sensor employing a flow rate detecting device which has a heating element and which is used for measuring the flow velocity or flow rate of a fluid according to a heat transfer phenomenon where a heat is transferred from the heating element or from a part heated by the heating element to the fluid. The present invention is applied to, for example, a flow rate sensor for use in measuring an intake air amount of an internal combustion engine.
2. Description of the Related Art
Japanese Unexamined Patent Publication Nos. 62-43522 and 4-2967 disclose known flow rate detecting devices each of which has a diaphragm structure and which is used in a flow rate sensor of such a type.
FIG. 24 is a plan view of a flow rate detecting device for use in a conventional flow rate sensor. FIG. 25 is a sectional view taken in the direction of arrows on line XXVxe2x80x94XXV of FIG. 24.
In the device shown in FIGS. 24 and 25, a plate-like substrate 1 is constituted by a silicon substrate about 0.4 mm thick. A 1-xcexcm-thick insulative supporting film 2 made of silicon nitride is formed on a surface of the plate-like substrate 1 by performing a method, such as sputtering or CVD. Moreover, a heating element 4 constituted by a thermo-sensitive platinum resistor film is formed on the supporting film 2. This heating element 4 is configured in the following process in such a manner as to form a current path. Namely, a 0.2-xcexcm-thick thermo-sensitive platinum film is first formed on the surface of the plate-like substrate 1 by using a vapor deposition or sputtering method. Then, patterning is performed on this thermo-sensitive resistor film by employing a photolithographic method and a wet (or dry) etching method. Furthermore, a fluid temperature detecting element 5 similarly constituted by a thermo-sensitive platinum resistor film is formed on the supporting film 2 apart from the heating element 4. This fluid temperature detecting element 5 is constructed in a process similar to the process of forming the heating element 4. First, a 0.2-xcexcm-thick thermosensitive platinum film is formed on the surface of the plate-like substrate 1 by using a vapor deposition or sputtering method. Subsequently, patterning is performed on this thermo-sensitive resistor film by performing a photolithographic method and a wet (or dry) etching method. Thus, this fluid temperature detecting element 5 is formed in such a way as to form a current path. Moreover, an insulative protective coat 3 is formed on the heating element 4 and the fluid temperature detecting element 5 by producing a 1-xcexcm-thick film made of silicon nitride through the sputtering or CVD method.
This heating element 4 is connected through connection patterns 9a and 9b and lead patterns 7a and 7d to electrodes 8a and 8d for connecting the flow rate detecting device to an external circuit. Further, the fluid temperature detecting element 5 is connected through lead patterns 7b and 7c to electrodes 8a and 8d for connecting the flow rate detecting device to an external circuit. Electrodes 8a to 8d are electrically connected to an external circuit by a method, such as a wire bonding. Thus, the protective coat 3 is removed from the electrodes 8a to 8d. 
Moreover, a cavity 13 is formed under a portion in which the heating element 4 is formed. Thus, a diaphragm 12 for detecting a flow rate is configured. Namely, a back-surface protecting coat 10 is first formed on the back surface (that is, a surface opposite to the surface on which the supporting film 2 is formed) of the plate-like substrate 1. Subsequently, the back-surface protecting coat 10 is partly removed at a place on the back surface side of the region, on which a heating element 4 is formed, by performing a photolithographic method. Thus, an etching hole 11 is formed. Thereafter, a part of the plate-like substrate 1 is removed by performing, for example, alkali etching on the plate-like substrate 1 exposed from the etching hole 11, so that the cavity 13 is formed. Consequently, the diaphragm portion 12 for detecting a flow rate is formed.
A flow rate detecting device 14 configured in this manner is arranged so that the diaphragm 12 for detecting a flow rate is exposed to a flow of a fluid to be measured. Incidentally, in these figures, arrows 6 indicate the direction of flow of the fluid to be measured.
Meanwhile, the flow rate detecting device 14 is shaped like a plate, as described above. In the case where the diaphragm 12 is placed in such a manner as to face the direction of flow of the fluid to be measured, a wind pressure is applied to the diaphragm 12, so that a failure of the diaphragm 12 is caused at a high flow rate. Further, dust contained in the fluid to be measured deposits on a diaphragm portion, with the result that drift in the flow rate detecting characteristics occurs. In such cases, the plate-like flow rate detecting device 14 is placed almost parallel to or at a predetermined angle with respect to the direction of flow of the fluid to be measured.
Furthermore, in the case that the plate-like flow rate detecting device 14 is placed almost parallel to or at a predetermined angle with respect to the direction of flow of the fluid to be measured, disturbance in flow of the fluid to be measured occurs in the vicinity of the cavity 13. Moreover, unevenness in the shape of the front edge portion of the flow rate detecting device 14, which is caused due to the chipping thereof, results in variation in flow of the fluid to be measured. This variation in the flow of the fluid to be measured, which is caused in the vicinity of the heating element 4, leads to reduction in accuracy of the flow rate detecting characteristics.
Thus, there has been previously proposed a flow rate sensor in which the flow-rate detecting device 14 is placed in a recess portion 18 provided in a plate-like supporting element 16, as illustrated in FIG. 26, to thereby prevent disturbance in flow of the to-be-measured fluid from occurring in the vicinity of the cavity 13, and in which the upstream-side end portion of the supporting element 16 is formed in an arcuated shape, thereby straightening the flow of the to-be-measured fluid and reducing variation in flow of the to-be-measured fluid, which would occur owing to unevenness in the shape of the front edge portion of the flow rate detecting device 14.
FIG. 26 is a perspective view of a primary part of the conventional flow rate detecting device.
In the device of FIG. 26, the supporting element 16 is shaped like a plate and attached to a base member 20. Further, the recess portion 18, whose perimeter is a little longer than that of the flow rate detecting element 14, is provided in a surface portion of the supporting element 16. The flow rate detecting device 14 is disposed in the recess portion 18 so that the top surface of the flow rate detecting device 14 is almost flush with the top surface of the supporting element 16. Moreover, the electrodes 8a to 8d of the flow rate detecting device 14 are electrically connected to leads 17, which are disposed in the base member 20, through wires 19. Furthermore, a cover 21 is attached to the base member 20, so that the electrodes 8a to 8d and the wires 19 are protected by the cover 21.
In the case that the fluid to be measured flows only on the surface of the flow rate detecting device 14 in a low flow rate range. However, in a high flow rate range, movement of the fluid to be measured occurs between the recess portion 18 provided in the supporting element 16 and the flow rate detecting device 14. Namely, as illustrated in FIGS. 27 and 28, a flow 22 of the to-be-measured fluid flowing on the surface of the flow rate detecting device 14 and a flow 23 thereof flowing between the recess portion 18 and the flow rate detecting device 14 are generated in the high flow rate range. Further, the flow 23 of the to-be-measured fluid is unstable, as compared with the flow 22 thereof. Thus, the accuracy in detecting a flow rate is deteriorated in the high flow rate range. Consequently, the conventional flow rate detecting device 14 has a drawback in that a flow rate measuring range is limited.
Then, to eliminate such a drawback, there has been devised a measure to completely closely stick or bond the back surface of the flow rate detecting device 14 to the recess portion 18 provided in the supporting element 16, thereby surely eliminating the aforementioned unstable flow 23 of the fluid to be measured
However, according to this measure, the cavity 13 is hermetically sealed. Thus, this measure has a drawback in that, when variation in pressure occurs in a flow path of the fluid, the deformation and breakage of the diaphragm 12 are caused owing to the difference between an internal pressure of the cavity 13 and an external pressure.
Moreover, when a flow rate sensor is manufactured, the step of bonding and fixing the flow rate detecting device 14 to the recess portion 18 provided in the supporting element 16 is a precise operation to be performed by preventing an adhesive, which has intruded into the cavity 13, from adhering to the diaphragm 12. Thus, this measure has another drawback in that the cost of manufacturing the flow rate sensor increases.
Furthermore, this measure facilitates the transmission of heat generated by the heating element 4 to the supporting element 16 through the plate-like substrate 1. Thus, this measure has still another drawback in that the flow rate detecting sensitivity of the sensor is degraded.
Further, to eliminate the aforementioned drawbacks, there has been proposed a supporting structure as disclosed in, for example, the Japanese Unexamined Patent Publication No. 9-26343.
In the supporting structure described in the Japanese Unexamined Patent Publication No. 9-26343, a recess portion 51 for accommodating the flow rate detecting device 14 in a surface portion thereof is provided in a sensor supporting element 50, as shown in FIG. 29. Moreover, a groove-like slot 52 is provided in the bottom part of the recess portion 51 in such a manner as to extend along the periphery of the flow rate detecting device 14. This slot 52 is provided in such a way as to extend outside a sensor area (namely, a flow rate detecting diaphragm area 13).
Further, the flow rate detecting device 14 is placed in the recess portion 51 so that the top surface of the device 14 is substantially flush with the sensor supporting element 50. Moreover, the flow rate detecting device 14 is fixed to the bottom surface of the recess portion 51 outside the diaphragm region 13 by an adhesive 53.
In this supporting structure, the to-be-measured fluid flows into the recess portion 51 from between the front edge surface (namely, the upstream edge surface) of the flow rate detecting device 14 and the upstream inner surface of the recess portion 51, as viewed in the direction of flow of the fluid. Subsequently, this fluid runs through the slot 52. Then, this fluid streams out from between the rear edge surface (namely, the downstream edge surface) of the flow rate detecting device 14 and the downstream inner surface of the recess portion 51, as viewed in the direction of flow of the fluid.
Thus, the unstable flow of the to-be-measured fluid flowing between the bottom portion of the recess portion 51 and the back surface of the flow rate detecting device 14 is reduced. Consequently, the deterioration in accuracy of flow rate detection in the high flow rate range is suppressed.
The supporting structure described in the Japanese Unexamined Patent Publication No. 9-26343 is constructed as described above. Therefore, the to-be-measured fluid led to the slot 52 sometimes goes round to a gap between the bottom part of the recess portion 51 and the cavity 13. Consequently, the fluid to be measured is not sufficiently prevented from flowing between the bottom part of the recess portion 51 and the back surface of the flow rate detecting device 14.
Further, the back surface of the flow rate detecting device 14 is placed close to the bottom part of the recess portion 51 except the part corresponding to the slot 52. Thus, most of heat generated by the heating element 4 is transmitted to the sensor supporting element 50. Consequently, the sensor employing this supporting structure has a drawback in that the flow rate detecting sensitivity of the sensor is deteriorated.
Additionally, in the case of forming a fluid temperature detecting device on the flow rate detecting device 14 by being combined therewith, the heat insulation between the flow rate detecting device 14 and the sensor supporting element 50 is insufficient because the flow rate detecting device 14 is placed close to the bottom part of the recess portion 51. Thus, the sensor employing this supporting structure has a drawback in that the detection response delay of the fluid temperature detecting device occurs.
The present invention is accomplished to eliminate the aforementioned drawbacks of the conventional sensor.
Accordingly, an object of the present invention is to provide a flow rate sensor with high flow rate measuring accuracy and good sensitivity.
To achieve the foregoing object, according to an aspect of the present invention, there is provided a thermo-sensitive flow rate sensor that comprises: a flow rate detecting device having a plate-like substrate, a heating element made of thermo-sensitive resistor film and formed on a surface of the plate-like substrate, and a cavity formed by removing a part of the plate-like substrate provided under the heating element to constitute a diaphragm for detecting a flow rate; and a supporting element having a recess portion for accommodating the flow rate detecting device formed on a top surface thereof, the supporting element being arranged so that the top surface thereof is in parallel with or at a predetermined angle with respect to a direction of flow of a fluid to be measured. In this thermo-sensitive flow rate sensor, a supporting face for supporting the flow rate detecting device is formed in the recess portion. Further, the flow rate detecting device is accommodated in the recess portion and is supported with and fixed to the supporting face so that the top surface of the flow-rate detecting device is nearly flush with the top surface of the supporting element. Moreover, a thin-plate-like member is attached to a back surface of the plate-like substrate of the flow rate detecting device in such a way as to close the cavity.
Further, according to another aspect of the present invention, there is provided a thermo-sensitive flow rate sensor that comprises: a flow rate detecting device having a plate-like substrate, a heating element made of thermo-sensitive resistor film and formed on a surface of the plate-like substrate, and a cavity formed by removing a part of the plate-like substrate provided under the heating element to constitute a diaphragm for detecting a flow rate; and a supporting element having a recess portion for accommodating the flow rate detecting device formed on a top surface thereof, the supporting element being arranged so that the top surface thereof is in parallel with or at a predetermined angle with respect to a direction of flow of a fluid to be measured. In this thermo-sensitive flow rate sensor, a plurality of first faces for supporting the flow rate detecting device are provided apart from one another in the recess portion, a second face facing a peripheral portion of the cavity and having a width being wider than that of the flow rate detecting device in the direction of flow of fluid to be measured is provided in the recess portion, and a third face, which does not face the cavity, is provided in the recess portion. Further, the second face is provided at a place that is deeper in a direction of depth of the recess portion than the plurality of first faces and the third face is provided at a place that is deeper in the direction of depth of the recess portion than the second face. Moreover, the flow rate detecting device is accommodated in the recess portion, is supported with the plurality of first faces and is fixed to at least one of the first faces so that the top surface of the flow rate detecting device is nearly flush with the top surface of the supporting element.
Moreover, according to still another aspect of the present invention, there is provided a thermo-sensitive flow rate sensor that comprises: a flow rate detecting device having a plate-like substrate, a heating element made of thermo-sensitive resistor film and formed on a surface of the plate-like substrate, and a cavity formed by removing a part of the plate-like substrate provided under the heating element to constitute a diaphragm for detecting a flow rate; and a supporting element having a recess portion for accommodating the flow rate detecting device formed on a top surface thereof, the supporting element being arranged so that the top surface thereof is in parallel with or at a predetermined angle with respect to a direction of flow of a fluid to be measured. In this thermo-sensitive flow rate sensor, a plurality of first faces for supporting the flow rate detecting device are provided apart from one another in the recess portion, a second face facing a peripheral portion of the cavity and having a width being wider than that of the flow rate detecting device in the direction of flow of fluid to be measured is provided in the recess portion, and a fourth face facing at least a part of the cavity is provided in the recess portion. Further, the second face is provided at a place that is deeper in a direction of depth of the recess portion than the plurality of first faces and the fourth face is provided at a place that is deeper in the direction of depth of the recess portion than the second face. Moreover, the flow rate detecting device is accommodated in the recess portion, is supported with the plurality of first faces and is fixed to at least one of the first faces so that the top surface of the flow rate detecting device is nearly flush with the top surface of the supporting element.